1. Field of the Invention
This invention relates to a high-temperature electrically regenerable system. More particularly it relates to an improved method for manufacture of a metal chalcogenide positive electrode providing simplicity and lower cost of manufacture with improved control of pore volumes and electrical conductivity, for use in a lithium-metal sulfide secondary cell.
2. Description of the Prior Art
Cells using lithium in the negative electrode with a molten or fused salt electrolyte have been extensively studied, utilizing either molten lithium or a solid alloy of lithium with aluminum as the negative electrode, a molten salt electrolyte such as the eutectic lithium and potassium chloride binary composition, a porous separator and a positive electrode of a metal sulfide such as copper sulfide, iron sulfide or nickel sulfide, or of elemental sulfur.
In order to develop a high energy density and the high electrical conductivity necessary for good performance in such a cell, it is desirable that the electrode material be in a finely divided state and that the particles of the solid electrode be contacted by an electrically conductive material.
Methods used previously have included the use of a carbon or graphite felt as a matrix which is then impregnated with sulfur or filled with the metal sulfide. The use of carbon foam and the use of a thermosetting binder which is then carbonized and/or graphitized to form a matrix holding the metal sulfide or other chalcogenide have been suggested.
These methods use a lattice or matrix of the porous carbon which is then impregnated with the positive electrode material, by mechanical means such as merely vibratory filling, by impregnating with a solvent carried mixture or by melting and subsequent impregnation, as in the case with elemental sulfur.
Repeated cycling of the lithium-sulfur cell results in a gradual loss of the sulfur by vaporization, reducing the feasibility of use of sulfur as a positive electrode material. Use of metal sulfides, which are non-volatile solids at the operational temperature, alleviates this difficulty but introduces another, namely that the operation of the cell with iron sulfide introduces the problem that as lithium sulfide forms, the iron is reduced in the cathodic reaction with a consequent volume increase, since the combination of lithium sulfide and iron has a higher volume than the iron sulfide. Consequent volume expansion and contraction put stresses on the matrix which, if it is rigid, will be destroyed eventually on cycling of the cell, making the cell useless.
This difficulty has been alleviated by filling a felt incompletely, leaving room within the lattice work for expansion; or by filling the felt or foam with a subliming type solid; or by impregnating the felt with a thermosetting resin which, on crosslinking, loses solvents and is then carbonized, producing a porous lattice or matrix with which the solid electrode material may expand.
An example of past practice using this approach is embodied in U.S. Pat. No. 4,011,374 to Thomas D. Kaun which discloses an electrode composition of thermosetting carbonaceous material, particles of the electrode material, and a sublimable solid, forming a moldable semi-solid. After molding, the paste is heated to cure the thermosetting resin and volatilize or sublime the solid which then forms a matrix with interstices into which the solid electrode material may expand. Saunders and Heredy, U.S. Pat. No. 3,925,098, disclose a porous felt body of resilient carbon or graphite fibers as a matrix which is impregnated with metal sulfide by a vibratory filling.